1. Technical Field of the Invention
The present invention relates to a motor-driven supercharger provided with an electric motor assisting a rotational drive of a compressor, in a supercharger driven by an exhaust gas of an internal combustion engine so as to compress and supercharge an intake air.
2. Description of the Related Art
In order to improve a performance of an internal combustion engine, there has been widely used a supercharger (also called as “turbocharger”) driven by an exhaust gas of the internal combustion engine so as to compress an intake air and supercharge. Further, there has been used a supercharger in which an acceleration response or the like is improved by incorporating an electric motor on the same axis as a shaft of a supercharger, and accelerating and assisting a rotational drive of a compressor. The supercharger having a motor-driven assist function by the electric motor is called as a motor-driven supercharger.
FIG. 1 is a view showing a skeleton structure of a conventional motor-driven supercharger 50. A turbine impeller 52 and a turbine housing 51A surrounding it are arranged in an exhaust passage side of the supercharger 50. The turbine housing 51A has a scroll chamber 63 formed around the turbine impeller 52, and the scroll chamber 63 is communicated with the turbine impeller 52 via an annular gas flow path 64.
Further, an exhaust port 65 having the same axis as the turbine impeller 52 is formed in a center portion of the turbine housing 51A.
In an intake passage side of the motor-driven supercharger 50, there are arranged a compressor impeller 53 and a compressor housing 51B surrounding it.
The compressor housing 51B has a scroll chamber 66 formed around the compressor impeller 53, and the scroll chamber 66 is communicated with the compressor impeller 53 via an annularly formed diffuser 67.
Further, an intake port 68 having the same axis as the compressor impeller 53 is formed in a center portion of the compressor housing 51B.
The turbine impeller 52 and the compressor impeller 53 are coupled by a shaft 54. The shaft 54 is rotatably supported by a bearing 55 built in a center housing 51C.
Further, the center housing 51C has an electric motor 58 having a rotor 56 coaxially coupled to the shaft 54, and a stator 57 arranged around the rotor 56 built-in.
In the motor-driven supercharger 50 structured as mentioned above, if an exhaust gas from an internal combustion engine (an engine) is introduced to the scroll chamber 63, the exhaust gas flows to the exhaust port 65 via the annular gas flow path 64 and rotates the turbine impeller 52 in the process of passing through the turbine impeller 52. Accordingly, at the same time when the compressor impeller 53 coupled to the turbine impeller 52 via the shaft 54 is rotationally driven, the rotational drive is assisted by the electric motor 58, thereby accelerating an air sucked from the intake port by the compressor impeller 53. The accelerated air is decelerated and pressurized in the process of passing through the diffuser 67 so as to be introduced to the scroll chamber 66, and is discharged from a discharge portion (not shown) so as to be supplied to the internal combustion engine.
In the motor-driven supercharger 50 mentioned above, the electric motor 58 is rotated at a high speed and generates self-heating due to a windage loss or an eddy current loss during an operation of the supercharger. Further, since a high-temperature exhaust gas flows through the turbine, the electric motor 58 becomes high temperature by a heat conduction from the turbine impeller 52 to the shaft 54 and from the shaft 54 to the rotor 56 of the electric motor 58.
If the electric motor 58 becomes high temperature, a permanent magnet in an inner portion thereof is demagnetized or an efficiency of the electric motor 58 is lowered. Further, in the case that the electric motor 58 becomes high temperature so as to reach an operation upper limit temperature, it is impossible to drive the electric motor 58. Accordingly, it is impossible to utilize a motor-driven assist function.
Accordingly, in the supercharger 50, as shown in FIG. 1, a turbine side cooling fluid flow path 60 is formed at a position in a turbine side of the center housing, and a peripheral position thereof is cooled by flowing the cooling fluid through the turbine side cooling fluid flow path 60, and the heat transmission to the electric motor side is suppressed.
Further, in the supercharger 50, an electric motor side cooling fluid flow path 61 is formed in such a manner as to surround the electric motor 58, and the structure is made such as to flow a cooling fluid 62 through the electric motor side cooling fluid flow path 61 and suppress a temperature increase of the electric motor 58 on the basis of a cooling operation.
The turbine side cooling fluid flow path 60 and the electric motor side cooling fluid flow path 61 are communicated by a communication path (not shown) formed within the center housing, and the common cooling fluid 62 flows through both the flow paths.
In this case, there is a method in which systems of cooling fluid flow paths are independently provided in the turbine side and the electric motor side, and independently flow the cooling fluids, however, in the light of easiness of design, manufacturing or the like, there has been widely employed a method of using a common cooling fluid in the turbine side and the electric motor side such as the supercharger 50.
Further, in the motor-driven supercharger mentioned above, a radiator water is generally used as the cooling fluid 62 flowing through the cooling fluid flow paths 60 and 61. In this case, the temperature of the cooling fluid 62 is about 80° C. to 120° C. In general, the motor-driven supercharger does not have a high necessity for always driving the electric motor, but is designed on the basis of an idea that it is sufficient to achieve the motor-driven assist effect for a certain period of time. Accordingly, it is designed such that a cooling capacity of the cooling fluid 62 and the temperature increase of the electric motor 58 do not balance. Even if the cooling fluid is flowed through the electric motor side cooling fluid flow path 61, it is impossible to lower the temperature of the electric motor 58, and the temperature of the electric motor 58 is increased.
In other words, the supercharger 50 is structured such as to suppress the temperature increase of the electric motor 58 by flowing the cooling fluid 62 through the electric motor side cooling fluid flow path 61, and elongate a time (a continuous operation time) until the electric motor 58 reaches the upper limit temperature (for example, 180° C.), at the operating time of the electric motor 58.
In this case, as the motor-driven supercharger mentioned above, various proposals are made, for example, the following patent documents 1 and 2.
Patent Document 1: Japanese Unexamined Patent Publication No. 2004-3420
Patent Document 2: Japanese Unexamined Patent Publication No. 2000-130176
As mentioned above, in the motor-driven supercharger, the radiator water is generally used as the cooling fluid flowing through the cooling fluid flow paths 60 and 61. In this case, the temperature of the cooling fluid is about 80° C. to 120° C.
Accordingly, in the case that the temperature of the electric motor 58 is higher than the temperature of the cooling fluid such as a rated operation time, there is an effect of suppressing the temperature increase of the electric motor 58.
However, in the case that the temperature of the electric motor 58 is lower than the temperature of the cooling fluid 62, just after starting the operation or in a partial load state in which the rotating speed is lower than the rated operation, the electric motor 58 is heated by the cooling fluid 62, and a temperature difference to the upper limit temperature of the electric motor 58 is reduced. In other words, in this case, the cooling fluid 62 shortens the continuous operation time of the electric motor 58, and achieves an absolutely inverse function to the original purpose.
Further, the applicant of the present invention filed the patent application about the motor-driven supercharger in which the cooling fluid flow path is provided at a position surrounding the electric motor and being adjacent to the diffuser, and having the cooling structure combining both the cooling functions of the electric motor and the diffuser.
In the motor-driven supercharger combining the cooling of the electric motor and the cooling of the diffuser as mentioned above, the cooling fluid having the high temperature heats the diffuser having the low temperature, in the partial load in which the compressor discharge temperature becomes lower than the cooling fluid temperature.
As a result, the discharge air of the compressor is heated, and there is a case that the efficiency of the compressor is lowered.